This invention relates in general to sound devices and in particular to a new and useful ear pad construction for earphones, particularly for stereophonic accoustic events.
The invention relates to an ear pad for earphones which in use embraces the user's ear and which comprises a tubularly formed, open-cell elastic foam material.
Similar ear pads are generally known (see e.g. AT-PS No. 362,433) and comprise in some cases coverings of a soft material slightly permeable to sound (e.g. thin leather or the like). Ear pads of this construction are usually used where good shielding against noise occuring outside the earphone is demanded. For the reproduction of acoustic events transmitted stereophonically they are not particularly suitable as their surfaces limiting the coupling space give rise to reflections in the coupling space, whereby the directional and distance hearing required in stereophonic reproduction and also the timbre are adversely affected.
In the so-called open earphones, where sound can emerge from the coupling space almost unhindered, the ear pad usually consists of open-cell elastic foam material of good sound permeability which generally has the form of a disk, and, in use, applies against the ear. From this, due to the good sound permeability of the pad, there necessarily results the effect of acoustic shortcircuit between front and back of the transducer diaphragm, since as a rule the back of the sound-generating diaphragm is not capsulated but radiates into the open without any substantial acoustic resistance. This results in a decline of the frequency response characteristic with decreasing frequency. To counteract this fact, the natural resonance of the sound-generating diaphragm has been moved into the region of the lowest frequencies, but without thereby obtaining a satisfactory result.
Apart from the endeavors to include in the frequency response of a good earphone also the frequencies lying in the lowest audible range into the essentially linear frequency response, the attempt has been made also to achieve compatitility between loudspeaker reproduction and earphone reception. This, however, has not been possible until now, or has at best accomplished only very insufficiently. While it was seen that the type of limitation of the coupling space which forms between the transducer diaphragm and the auditory canal when the earphone is worn is critical, a satisfactory solution has not been found despite this finding. Also the proposal aiming in that direction, of changing over from the free-field-corrected to the diffused-field-corrected earphone yielded only moderate success. In fact, it was not taken into account that it is possible with one ear alone to locate a sound source, especially if it is small (for instance a mosquito flying by) clearly as to direction and distance. The reason for this ability of the human ear is that the reflections on the external ear cause a comb filter effect, which by training leads to a surprising certainty of localization already at an early age, supported by the sense of sight. This effect, in the following referred to as near effect, is also the reason why, with the conventional headphones, compatibility between loudspeaker reception and earphone reception cannot be achieved, and why, therefore, all attempts to suppress the above described effect by influencing the electric signal supplied to the earphone, e.g. by a special form of the frequency response, by transit time delay or by adding reverberation, were doomed.
Proof that such a near effect exists can be furnished by a simple experiment. In fact, if one directs white noise in the form of a flat sound field against the ear from a distance of, for instance, 100 cm, and between the sound generator and the ear one moves an extremely thin, i.e. low-mass, loosely hanging large membrane (foil) a few microns thick toward the ear from a distance of about 30 cm. It is found that at a distance of only 5 cm between ear and membrane a change of timbre of the noise is noticeable, if a slight one. As the membrane is brought closer to the ear, for instance to about 2 cm and less, the change in timbre is quite strong., proving that a near effect exists. The cause of this effect is attributable to the pressure accumulation between the auricle or respectively the external auditory canal and the membrane, from which a disturbance of the aforementioned comb filter effect results.